(a) Field of the Invention
The present invention relates to a method of laser welding. More particularly, the present invention relates to a method of laser welding which can reduce a pin hole generated at an end portion of a welding portion when the laser welding is performed after steel plates are overlapped.
(b) Description of the Related Art
Generally, a spot welding is mainly used as a welding method for assembling components of a vehicle body, and an arc welding or a laser welding is used at welding portions to which the spot welding cannot be applied.
Particularly, since the laser welding has merits such that a filler metal is not required, a welding cycle is short due to fast welding speed, a heat input is low, a heat affected zone (HAZ) is small, and a deformation is small, as shown in FIG. 1, the laser welding is increasingly used for welding the components of the vehicle body instead of the spot welding and the arc welding.
A keyhole welding using energy reflection and absorption in a focal region of a laser beam is mainly used as the laser welding. The keyhole welding is done in the focal region at which the laser beam is collected to welding materials by a lens and energy reflection and absorption occur.
That is, the laser beam collides with the welding materials in the focal region where electromagnetic wave energy of the laser beam is collected, and collision energy is converted into heat energy such that keyhole of the electromagnetic wave occurs. Accordingly, the keyhole welding is performed. The keyhole welding is a welding performed by making small hole at a molten pool due to vapor pressure when laser welding.
A laser welding system shown in FIG. 1 welds steel sheets or plated steel sheets.
According to the laser welding system, a laser head 5 is mounted at a front end of an arm 3 of a robot 1, and the laser head 5 is connected to a laser oscillator 7 through a optical fiber.
The robot 1 is operated by a robot controller C, and the laser head 5 is moved along a welding portion W (referring to FIG. 3) of the welding material 9 by the robot 1. Therefore, the laser head 5 irradiates a laser beam LB and performs the welding process.
A conventional laser welding, however, is a keyhole welding of high power. Since the molten pool for filling up a keyhole positioned at an end portion of a welding pattern of the welding portion W is insufficient according to the keyhole welding, a pin hole PH having a micropore configuration is generated, as shown in FIG. 3. Such pin holes PH may deteriorate an appearance of the welding portion W and may be major causes of interfacial fractures at the welding portion W. Therefore, welding strength may be deteriorated.
Meanwhile, control factors which affect on a depth of the pin hole PH may be an output of the laser beam, welding speed, a defocusing amount, a width of the welding pattern, and so on. However, generation of the pin holes PH may not be basically prevented only by controlling such control factors.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.